Pressure vessel seal



A 1966 R. P. LEVEY, JR, ETAL 3,266,095

PRESSURE VESSEL SEAL Filed Dec. 2, 1963 INVENTORS. h P. Levey, Jr. BY L. Hudd/esfon ATTORNEY.

United States Patent 3,266,095 PRESSURE VESSEL SEAL Ralph P. Levey, .l'r., Oak Ridge, and Roy L. Huddleston,

Knoxville, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Dec. 2, 1963, Ser. No. 327,563 2 Claims. (Cl. 18-16) This invention relates generally to ultra-high pressure vessels and more particularly to an improved means for sealing the annular space between the piston and encompassing female die member in piston-cylinder type ultra-high pressure vessels.

Characteristic of prior art piston-cylinder type ultrahigh pressure vessels, has been the problem of maintaining an effective seal between the piston and the encompassing die member over wide pressure ranges. Such "a seal is critical in the successful operation of the pressure vessel inasmuch as leakage of fluid pressure-transmitting medium through the seal region prevents or reduces the build up of pressure in the die cavity as the piston advances therein. Such leakage effectively limits not only the magnitude, but also the duration of the attainable pressure in the die cavity.

Basically, the sealing problem in piston-cylinder type pressure vessels is due to the different inherent rates of expansion with pressure of the piston and die member. Specifically, the die member expands radially at a greater rate with increasing pressure in the die cavity than does the typical solid piston member. It is because of this unfavorable differential expansion that O-rings tend to fail above 100,000 p.s.i., where the clearance between the solid piston and the encompassing die member becomes so great that the Oring seal is incapable of supporting, due to high shear stresses, the resulting force exerted on the O-ring seal by the fluid pressure-transmitting medium. The force exerted on the O-ring is approximately equal to the pressure developed in the die cavity times the total cross-sectional area of the piston to die member clearance or gap in the vicinity of the O-ring. This force varies, therefore, as the product of two dependent variables which increase simultaneously during a pressing operation.

In an attempt to increase the attainable pressure in a piston-cylinder type pressure vessel, interference fits between the piston and the encompassing die member have been tried. Although such a scheme has been successful in attaining the primary objective of higher pressures, it has concomitantly introduced deleterious side effects such as the requirement that large thrust forces be exerted on the piston member during insertion into and removal from the die cavity. These large thrust forces preclude hand assembly and disassembly of the pressure vessel. This type of system is also inherently ineflicient because a large fraction of the energy used in the pressing operation is required merely to overcome the frictional force between the piston and die member. Piston-cylinder pressure vessels of this type also tend to wear rapidly, thereby limiting the number of pressing operations which can be performed with any given vessel.

It is, therefore, a general object of the invention to provide, in a piston-cylinder type pressure vessel, a piston member, at least a portion of whose wall expands at a rate substantially equal to the rate of expansion of an encompassing die member during a pressing operation.

Another object of the invention is to provide, in a piston-cylinder type pressure vessel, a piston member which maintains a close piston to die member fit over a wide range of die cavity pressures without interference between piston and die member at low pressures.

Patented August 16, 1966 Another object of the invention is to provide a pistoncylinder type pressure vessel capable of operating at pressures greater than 100,000 p.s.i. wherein the piston and die member can be assembled and disassembled by hand.

Other objects of the invention will become apparent from an examination of the following description of the invention and the appended drawing which is a diagrammatic cross-section of a piston-cylinder type pressure vessel utilizing the subject invention.

In accordance with the present invention, an improved piston-cylinder type pressure vessel has been provided, wherein the piston has a wall portion which expands in response to pressure changes in the die cavity at substantially the same rate as the encompassing die member. The piston member is provided with a coaxial annular wall integrally aflixed to its end inserted into the die member. An O-ring, encircling the outer surface of the annular wall adjacent the free end thereof, provides a sealing contact between the female die member and the piston member. As pressure is generated in the die cavity, the annular wall of the hollowed piston expands radially at substantially the same rate as the wall of the die member, thereby minimizing the gap to be sealed between the piston and die member.

In the drawing, a male piston member 1 is shown partially inserted into the central cavity portion of a female die member 2. The end of the piston member 1 outside of the die cavity is provided with an enlarged flange 3 upon which force is exerted during a pressing operation to drive the piston member into the central die cavity. A pressure chamber 4, acting as a receptacle for a specimen to be pressed and a pressure transmitting medium, is that portion of the central die cavity which lies below piston member 1 and above stepped plug 5. Stepped plug 5, driven into the die cavity in an interference fit, sealably closes the lower end thereof. Piston member 1, at its end which is inserted into the central cavity, is provided with a coaxial annular wall 6 which defines a concentric longitudinal cavity 7. A neoprene O-ring 8, residing in a peripheral groove 9, encircles the annular wall 6 adjacent to the free end thereof.

As indicated in the drawing, the wall 6 defining cavity 7 is relatively thin. Furthermore, cavity 7 is open to pressure chamber 4 and thus experiences the same pressures as are generated therein. The function of the hollowed end of the piston member 1 is to provide a piston section whose radial growth rate in response to pressure changes in pressure chamber 4, is the same as the radial growth rate of the wall 10 of chamber 4. The radial growth rate of annular Wall 6 is greater than the radial growth rate of the prior solid type piston due to the fact that the wall 6 is exposed to a net outward radial pressure rather than the axial pressure only as experienced by the end surfaces of solid type pistons. The radial expansion of wall 6, arising in the same manner and in response to the same pressures as cause radial expansion of the pressure chamber walls, can be tailored, through the selection of a proper wall thickness, to expand radially at substantially the same rate as the walls 10 of pressure chamber 4. With such a matching radial growth rate the piston to encompassing die member gap can be maintained sufiiciently small so as to enable an O-ring seal to operate even at high pressures.

O-ring 8 seals the piston to die member gap and thereby prevents leakage of the pressure-transmitting fluid from pressure chamber 4 into the piston to die member gap above the O-ring. The O-ring, therefore, restricts the portion of the piston member exposed to the high pressure fluid of chamber 4 to the inner portion of wall 6 defining cavity 7 and to that outer portion of wall 6 which lies below O-ring 8. This arrangement causes a large pressure dilferential to exist across wall 6 above the O-ring; the pressure differential increasing with increasing pressure in pressure chamber 4. Wall 6, whose radial growth is dependent on the pressure differ ential thereacross as determined by the pressure in chamber 4, is thus tailored to expand radially at substantially the same rate as the wall of die member 2; thereby insuring a minimum piston to die member clearance which the O-ring can effectively seal at high pressures without failure.

To insure continued sealing between the piston member 1 and die member 2 as the pressure within chamber 4 increases from atmospheric to ultra-high pressures exceeding 100,000 p.s.i., the thickness of annular wall 6 must be chosen so as to enable the wall 6 to expand radially at or about the same rate as the radial expansion experienced by die member 2. The diametral ratio of wall 6 to be used with a simple cylindrical type die member may be determined by means of the following formula:

where Kp=ratio of outside to inside diameter of the piston member.

Kc=ratio of outside to inside diameter of the female die member.

Ec elastic modulus of the die member.

Ep=elastic modulus of the piston.

Mc Poissons ratio for the die member.

The above equation assumes a cavity sufliciently deep so that the bending moment, at the point where the cavity wall merges into the solid portion of the piston 1, exerts a negligible effect on the expansion of annular wall 6 in the region of the O-ring. Secondary stress effects, such as the axial stress in wall 6 have not been considered in the above equation. It is noted that in pistoncylinder type pressure vessels which have radially supported die members, annular wall 5 can be made thicker than indicated by the above equation because the radial growth of the die member is reduced by radial support.

In a typical piston-female die member arrangement the following values are illustrative of the relative wall thicknesses. Assuming a die member of 6 inches outside diameter and a 0.75 inch inside diameter, a modulus of elasticity of 30x10 for the cylinder and piston member, and a Poissons ratio of 0.29 for the die member, the inside diameter of the annular wall 6 of the piston member will be 0.473 inch and the outside diameter of the piston will be 0.75 inch. The value of the outside diameter of the piston need only be reduced a few thousandths of an inch to permit manual insertion and extraction of the piston into the 0.75 inch opening of the female die member 2.

In a typical pressing operation, the sample to be pressed is placed into the lower end of the die cavity. Pressure-transmitting fluid is poured into the cavity so as to cover the sample and almost fill the die cavity. The piston member 1 is then inserted into the cavity manually as far as possible. Force is then applied to the enlarged flange 3 of piston 1 so as to force piston member 1 into the die cavity thereby causing a pressure rise in chamber 4.

Since many modifications of and deviations from the embodiments disclosed herein may be made without departing from the spirit and scope of the present invention, the foregoing illustrative description of the embodiment should not be interpreted in a limiting sense. The invention should be limited only by the scope of the claims appended hereto.

We claim:

ll. In a pressure vessel comprising a female die member, a fluid pressure-transmitting medium disposed within said die member, and a piston member slid-ably inserted in said die member, the improved sealing means for preventing passage of said pressure-transmitting medium between said piston member and said die member comprising, in combination, a coaxial annular wall, said wall integrally aflixed to said piston member at that end of said piston member inserted in said die, said annular wall having an outer radius substantially equal to the internal radius of said die, and an O-ring encircling said annular wall adjacent the free end thereof, said fluid, increasing in pressure upon insertion of said piston into said die member, expanding said annular wall in a substantially radial direction to maintain said O-ring in sealing contact with said die member.

2. In a pressure vessel comprising a cylindrical female die member, a fluid pressure-transmitting medium disposed within said die member, and a piston member slidably inserted into said die member; the improved sealing means for preventing passage of said pressure-transmitting medium between said piston member and said die member comprising, in combination; a coaxial annular wall integrally afiixed to the end of said piston member inserted into said die member, and an O-ring encircling the outer surface of said annular wall adjacent the free end thereof, said annular wall having an outer diameter substantially equal to the inner diameter of said die member and an inner radius determined by the following equation:

E0 V 2 7 Kc +1 where:

Kp=the ratio of the outer to inner diameters of said annular wall.

Kc=the ratio of the outer to inner diameters of said female die member.

Ec the modulus of elasticity of said die member.

Ep=the modulus of elasticity of said piston member.

Mc Poissons ratio for said die member.

No references cited.

I. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner. 

1. IN A PRESSURE VESSEL COMPRISING A FEMALE DIE MEMBER, A FLUID PRESSURE-TRANSMITTING MEDIUM DISPOSED WITHIN SAID DIE MEMBER, AND A PISTON MEMBER SLIDABLE INSERTED IN SAID DIE MEMBER, THE IMPROVED SEALING MEANS FOR PREVENTING PASSAGE OF SAID PRESSURE-TRANSMITTING MEDIUM BETWEEN SAID PISTON MEMBER AND SAID DIE MEMBER, COMPRISING IN COMBINATION, A COAXIAL ANNUAL WALL, SAID WALL INTERGRALLY FFIXED TO SAID PISTON MEMBER AT THAT END OF SAID PISTON MEMBER INSERTED IN SAID DIE, SAID ANNUAL WALL HAVING AN OUTER RADIUS SUBSTANTIALLY EQUAL TO THE IN TERNAL RADIUS OF SAID DIE, AND AN O-RING ENCIRCLING SAID ANNUAL WALL ADJACENT THE FREE END THEREOF, SAID FLUID, INCREACING IN THE PRESSURE UPON INSERTING OF SAID PISTON INTO SAID DIE MEMBER, EXPENDING SAID ANNUAL WALL IN A SUBSTANTIALLY RADIAL DIRECTION TO MAINTAIN SAID O-RING IN SEALING CONTACT WITH SAID DIE MEMBER. 